Conventional floating gate flash memory types of EEPROMs (electrically erasable programmable read only memory), utilize a memory cell characterized by a vertical stack of a tunnel oxide (SiO2), a polysilicon floating gate over the tunnel oxide, an interlayer dielectric over the floating gate (typically an oxide, nitride, oxide stack), and a control gate over the interlayer dielectric positioned over a crystalline silicon substrate. Within the substrate are a channel region positioned below the vertical stack and source and drain diffusions on opposing sides of the channel region.
The floating gate flash memory cell is programmed by inducing hot electron injection from the channel region to the floating gate to create a non volatile negative charge on the floating gate. Hot electron injection can be achieved by applying a drain to source bias along with a high control gate positive voltage. The gate voltage inverts the channel while the drain to source bias accelerates electrons towards the drain. The accelerated electrons gain 5.0 to 6.0 eV of kinetic energy which is more than sufficient to cross the 3.2 eV Si—SiO2 energy barrier between the channel region and the tunnel oxide. While the electrons are accelerated towards the drain, those electrons which collide with the crystalline lattice are redirected towards the Si—SiO2 interface under the influence of the control gate electrical field and gain sufficient energy to cross the barrier.
Once programmed, the negative charge on the floating gate increases the threshold voltage of the FET characterized by the source region, drain region, channel region, and control gate. During a “read” of the memory cell, the magnitude of the current flowing between the source and drain at a predetermined control gate voltage indicates whether the flash cell is programmed.
More recently dielectric memory cell structures have been developed. A conventional dielectric memory cell 10 is shown in cross section in FIG. 1 and is characterized by a vertical stack of an insulating tunnel dielectric layer 12, a charge trapping dielectric layer 14, an insulating top oxide layer 16, and a polysilicon control gate 18 positioned on top of a crystalline silicon substrate 15. Within the substrate 15 are a channel region 17 positioned below the vertical stack and source diffusion 19 and drain diffusion 23 on opposing sides of the channel region 17. This particular structure of a silicon channel region 22, tunnel oxide 12, nitride 14, top oxide 16, and polysilicon control gate 18 is often referred to as a SONOS device.
Similar to the floating gate device, the SONOS memory cell 10 is programmed by inducing hot electron injection from the channel region 17 to the nitride layer 14 to create a non volatile negative charge within charge traps existing in the nitride layer 14. Again, hot electron injection can be achieved by applying a drain-to-source bias along with a high positive voltage on the control gate 18. The high voltage on the control gate 18 inverts the channel region 17 while the drain-to-source bias accelerates electrons towards the drain region 23. The accelerated electrons gain 5.0 to 6.0 eV of kinetic energy which is more than sufficient to cross the 3.2 eV Si—SiO2 energy barrier between the channel region 17 and the tunnel oxide 12. While the electrons are accelerated towards the drain region 23, those electrons which collide with the crystalline lattice are re-directed towards the Si—SiO2 interface under the influence of the control gate electrical field and have sufficient energy to cross the barrier. Because the nitride layer stores the injected electrons within traps and is otherwise a dielectric, the trapped electrons remain localized within a drain charge storage region 13 that is close to the drain region 23 (or in a source charge storage region 11 that is close to the source region 19 if a source to drain bias is used) from which the electrons were injected. As such, the SONOS device can be used to store two bits of data, one in each of the charge storage regions 11 and 13, per cell and are typically referred to as dual bit SONOS devices.
A problem associated with dual bit SONOS structures is that the trapped charge in the drain and source charge storage regions 13 and 11 has a finite spatial distribution that peaks at the drain region 23 and source region 19 respectively and a portion of the charge distribution will spread into the area between the source charge storage region 11 and the drain charge storage region 13. The spread charge effects the threshold voltage during the read cycle. The charge that accumulates between the source charge storage region 11 and the drain charge storage region 13 is difficult to remove utilizing the hot hole injection erase mechanism. Additionally, charge spreading become more problematic over the lifetime of operation of the device. Each program/erase cycle, may cause further spread of electrons into the area between source charge storage region 11 and the drain charge storage region 13. The problem is further compounded by the continued decrease in the size of the semiconductor devices, which calls for nitride layers with less area separating the two charge storage regions 11 and 13.
A need exists in the art for a dual bit memory cell structure which does not suffer the disadvantages discussed above.